The ultrasonic absorption and velocity of dilute solutions of methyl, chloro‐, and bromocyclohexane in xylene have been measured over the range 0.6 to 25 Mc and 65° to 95°C. The frequency dependence of the absorption in all cases is that of a single relaxation process. The concentration dependence of the relaxation parameters is consistent with the mechanism being the interchange of axial and equatorial chair isomers, while the temperature dependence yields a difference in enthalpy and entropy between isomers of methyl cyclohexane of 3.54±0.7 kcal/mole and 4.04±1.1 cal/mole °C, respectively, and an activation energy for the conversion of axial into equatorial isomers of 10.94±0.8 kcal/mole for methyl cyclohexane and 12±2 kcal/mole for chloro‐ and bromo‐cyclohexane. There is no significant change in these values for methyl cyclohexane when the solvent is changed to nitrobenzene orn‐butyl alcohol, indicating that intermolecular processes do not make an appreciable contribution, and that the difference in volume between isomers can be neglected in the calculations. The conversion of axial into equatorial isomers is found to be uni‐molecular, and a mode of vibration of the molecule similar to either theA1gorEumodes of cyclohexane is suggested for the path of this reaction.